1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine, and more specifically to an air-fuel ratio control system in which the control characteristics are changed depending on the engine load.
2. Description of Background Information
Internal combustion engines provided with a three-way catalytic converter in the exhaust system for the purification of the exhaust gas are sometimes equipped with an air-fuel ratio feedback control system by which the air-fuel ratio is controlled around a stoichiometric value according to the exhaust gas concentration and the operating conditions of the engine. This is because an optimum operation of the three-way catalytic converter is enabled when the air-fuel ratio of the mixture supplied to the engine is at around the stoichiometric air-fuel ratio (14.7 for example).
Generally, in the case of such air-fuel ratio control systems, an oxygen sensor is provided in the exhaust system of the engine so that whether the actual air-fuel ratio is rich or lean is detected by comparing the output signal level of the oxygen sensor with a predetermined reference level corresponding to the stoichiometric air-fuel ratio. If the result of the comparison indicates rich mixture, the air-fuel ratio of the mixture is controlled to the lean side and if the result of the comparison indicates lean mixture, the air-fuel ratio is controlled to the rich side.
In the case of internal combustion engines using a carburetor as the means for supplying the mixture into cylinders, the air-fuel ratio of the mixture supplied by means of the carburetor is controlled at around the stoichiometric value in accordance with the variation of the engine load. Specifically, when the engine load is high, the air-fuel ratio becomes richer so that the reduction of the engine output power is prevented, because high output power is required during the high load operation of the engine. On the other hand, when the engine load is medium or low, the air-fuel ratio is made lean because the fuel economy is preferred rather than the engine output power under such operating conditions of the engine.
As the air-fuel ratio control system, a control apparatus of the type of air intake side secondary air supply system is known in which an air intake side secondary air supply passage communicating with the downstream side of the throttle valve of the carburetor is provided and the air-fuel ratio control is performed by varying the amount of the secondary air flowing through the air intake side secondary supply passage. In such air-fuel ratio control systems, the air-fuel ratio becomes richer during the high load operating condition of the engine because the opening angle of the throttle valve is increased during the high load operation of the engine, and the magnitude of the vacuum on downstream side of the throttle valve is reduced, and which in turn causes the reduction of the flow of the air intake side secondary air. On the other hand, when the engine load is medium or low, the air-fuel ratio becomes leaner since the opening angle of the throttle valve is small under such an operating condition of the engine, and the magnitude of the vacuum on the downstream side of the throttle valve is increased, which in turn causes the increase of the flow of the air intake side secondary air.
In the case of the conventional fuel supply systems, it was often the case due to the above reason that the air-fuel ratio fluctuates with change in the engine load outside a normal range of the air-fuel ratio in which the operation of the three-way catalytic converter for purifying the exhaust gas is effective. Therefore, it has been required to present a system in which the air-fuel ratio of the mixture is always controlled to the stoichiometric value regardless of the change of the engine load.